Keep in mind that the default zoom level on your SP is 150%. This will extend to your monitor until MS gets around to fixing that. This means that things will be BIG on your external monitor. On a 27" monitor they will be REALLY big. I went with a 21.5" HP IPS display from BestBuy that was on sale for $129 and am quite happy with it. You can find them even cheaper on Amazon
. One thing to keep in mind when monitor shopping is do you want reflective of non-reflective surface. Non-reflective is good if you have lights behind you as they won't reflect off the screen. Reflective will give you a brighter crisper picture.
Couple options to make things less big
on your external monitor:
1) Set your zoom at 125%. Makes things a bit small on the SP, but better on the external monitor.
2) Change the resolution of your SP to 1600 x 900 and set the zoom to 125%. You probably won't notice any degradation in SP image quality and 1600 x 900 x 125 gives you same screen real estate at 1920 x 1080 x 150.